Traffic speed data and travel time estimates are becoming widely available from commercial vendors. However, these are not sufficient for a proper performance evaluation and active management of a transportation network infrastructure, since effective road network planning and traffic operation requires the knowledge of traffic flows. Information currently available in the market does not properly represent accurate traffic flow data.
Traffic flow can be measured from annual average daily traffic (AADT) figures, which are a measure used primarily in transportation planning and transportation engineering. AADT is the total volume of vehicle traffic of a roadway for a year, divided by 365 days, and provides a simple measurement for how busy a roadway is in terms of such volume. Each year, every state in the United States submits a Highway Performance Monitoring System (HPMS) report that contains transportation network information (roadway links, and their shapes, comprising each state's network) with assigned AADT values. HPMS links are bidirectional, and thus the AADT volume in each HPMS report is a sum of daily flows in both directions for each roadway link. In general, AADT values can be roughly divided by 2 in order to obtain daily flow for a single direction. However, determining average daily flows to describe traffic volumes on a particular day is also not enough of a sufficient measure of traffic flow.
That is because traffic volume at any given location depends on the season, day of week, and other factors such as whether the day is holiday or not, and the time of day. FIG. 2 shows the dynamics of monthly flows collected from a vehicle detector station (VDS) representing a fixed location of California's Interstate 5 for both directions, North and South. From FIG. 2, it is evident that the monthly flows are nearly equal in both directions, but this plot does not account for any of the other characteristics noted above.
Traffic speed data is derived from many different sources, such as for example from roadway sensors such as radar and video systems. Traffic speed data may also be derived from data collected from providers of Global Positioning Systems (GPS) services and the like. Data from GPS services is sold in bulk, by the number of data points per day or per month, and may be packaged in different ways. For example, GPS probe data may be in the form of “raw” or unprocessed probe data points, or in the form of processed probe data that reflects traffic speed on a roadway network. Regardless of the source, the traffic speed data derived from this collected GPS data provides an indication of traffic speed at a given point in time, but does not produce an indication of traffic flow. Therefore, more is needed for effective planning and operation of traffic in a transportation network.
Accordingly, traffic flow determinations purely from available traffic speed data or from available AADT are insufficient, by themselves, to provide an accurate indication of traffic flows and reasons for them. They are also insufficient to provide an accurate estimation of hourly traffic flows. Knowledge of both traffic flows generally, and estimates of hourly traffic flows, is very helpful in transportation network infrastructure planning management.